Along with a recent development of multimedia presentation, it is required to process a large amount of information including video information. It is also required for a storage device storing therein the information to have a larger capacity. In the field of storage device that stores therein a high-image-quality video information in particular, a capacity larger than the capacity of the current DVD is desired. However, for achieving a larger storage capacity, it is needed for an optical disk drive or HDD unit to increase the storage density. Thus, for increasing the storage density, it is a crucial issue to reduce the error rate and assure the reliability. To address this issue in the optical disk, there are roughly three approaches that have been investigated: a media composition approach; an optical approach; and a signal processing approach. The following description relates mainly to the signal processing approach.
The optical disk drive irradiates a laser beam focused by an optical element onto a disk medium, to detect information based on the brightness and darkness or polarization of the light reflected therefrom. Since the focused beam spot is infinite and a smaller diameter thereof enables a higher density recording/reproducing, the optical approach for reducing the beam spot has been developed. The spot diameter is inversely proportional to the NA (numerical aperture) of an objective lens, and is proportional to the wavelength, λ, of the laser beam. Therefore, a larger NA and a smaller λ can provide a smaller spot diameter. However, a larger NA reduces the depth of focus, to necessitate a smaller distance between the disk surface and the lens, and thus there is a limit thereon. On the other hand, although a shorter wavelength laser has a problem on the stability upon lasing at a higher output power and on the lifetime thereof, the reduction of the wavelength has been gradually advancing from an infrared laser (λ=780 nm) for a CD, through a red laser (λ=650 nm) for a DVD, to a blue laser (λ=405 nm) for the next-generation DVD.
Here, the frequency characteristic of a transmission path between the optical head and the disk medium is in the form of a LPF (low-pass filter) in which the gain of a higher frequency range reduces due to the finite beam spot. Accordingly, a rectangular wave, if recorded, will have a dull waveform. In addition, a higher recording density, if employed, incurs an intersymbol interference wherein a waveform to be read at a specific time instant interferes with another waveform to be read at another time instant. The intersymbol interference makes it difficult to reproduce a recorded mark having a length smaller than a specific length. On the contrary, in consideration of the case of a longer recorded mark, the resultant reduction in the frequency of outputs of the phase information, which are used for extracting a synchronizing clock, may incur a loss of synchronization, and accordingly, the length of the recorded mark must be limited to below a specified length.
Due to the reasons as described heretofore, the signal processing approach is such that the data to recorded onto the optical disk is encoded for recording. In particular, a RLL code (run-length-limited code) that restricts the interval between code inversions is mostly used, and especially ETM (eight to twelve modulation), EFM (eight to fourteen modulation), (1,7)RLL, 8/16 codes etc. are typically used. The minimum run-length in the EFM-modulated code used for the CD and the 8/16-modulated code employed for the DVD, among others, is 2 (d=2), whereas the maximum run-length in the (1,7)RLL and ETM-modulated codes is “1”. The ETM is (1,10)RLL code in which the code rate is ⅔ similarly to (1,7) RLL, as described in “Eight to Twelve Modulation Code for High Density Optical Disk”, Kinji Kayanuma et al., International Symposium on Optical Memory 2003, Technical Digest pp. 160-161, Nov. 3, 2003. This features a restriction of the number of continued shortest marks in a row and a compression performance of the DC (direct current) component.
There is a technique referred to as waveform equalization. This reduces the error rate by inserting a reverse filter that removes the intersymbol interference. Since this equalization emphasizes the high-frequency component of the readout signal, the intersymbol interference can be suppressed; however, there may be a case where the high-frequency component of noise is also emphasized to thereby degrade SNR (signal to nose ratio) of the readout signal. In particular, in the case of higher recording density, the degradation of SNR caused by this waveform equalization is the main factor of occurring of an error in the detection data. A PR (partial response) equalization is one of the waveform equalization techniques that intentionally generates the intersymbol interference. Usually, it does not intensify the high-frequency component, whereby the degradation of SNR can be suppressed.
On the other hand, there is a maximum-likelihood detection technique among the effective detection techniques. This technique is one that raises the detection performance by selecting a pattern that provides a least square mean value of errors out of all the possible time-series patterns. Note that an algorithm referred to as the Viterbi algorithm is generally used to achieve the processing by performing a progressive path selection, because the above processing is generally difficult to achieve in the practical circuitry due to restriction of the circuit scale and operational speed.
A detection technique that combines the Viterbi detection with the above PR equalization is referred to as PRML (partial-response maximum-likelihood) technique, and can detect data while performing a sort of error correction. The PR equalization allows the readout signals to have correlation therebetween in the time direction. Thus, the data series sampled from the readout signal have only limited specific state transitions. By comparing the limited state transitions against the data series of the actual readout signal including noise to select a state transition that is most likely, occurring of an error in the detection data can be suppressed. The PRML detection technique using the ETM code and PR (1,2,2,2,1) channel is described in “Development of HD DVD technique (recording technique)” by Ogawa, Honma et al., Institute of Image Information and Television Engineers Technical Report, ITE Technical Report, Vol. 28, No. 43, PP. 17-20 MMS2004-38, CE2004-39 (July 2004). This technique achieves a wider detection margin during reproduction of a high density recording.
For improving the detection performance in the Viterbi detection, it is needed to match the frequency characteristic of the reproduced channel with the specific PR equalization characteristic. In this case, a PR equalization characteristic that is most close to the reproduced channel is selected, and in general, the frequency characteristic is corrected using the waveform equalizer, to be close to the PR characteristic as much as possible. As a technique for adaptively correcting the time-dependent degradation of the signal to thereby improve the detection performance, there is an automated equalization or adaptive equalization technique. A successive adaptive equalization algorithm is described in “Fundamental of Current Information Communication” by Shuzo Saito etc. from OHM corp., Dec. 20, 1992, pp 212-217, and includes typically “Zero Forcing Technique” and “Mean Square Technique” etc. The adaptive equalization technique has the advantage that initializing adjustment of the device is not needed and so on, to operate with a higher effectiveness.
In the mean time, a DC fluctuation of the readout signal degrades the detection performance in other detection techniques as well as the Viterbi detection. Usually, for compensating this degradation, the detection processing is performed after using a HPF (high-pass filter) etc. to correct a DC deviation. However, if there is a deviation in the symmetry of the readout signal, passing through the HPF does not match the polarity inversion level with the zero reference level. For the case where the readout signal is subjected to a threshold detection using a zero threshold value, a technique referred to as automated slicer is used wherein the threshold value is controlled by integrating the binary data to allow the duty thereof have an average of zero. This technique is described as an example of the conventional technique in Patent Publication JP-1995-296386A.
The above automated slicer will be described with reference to FIG. 9. A readout signal detected by an optical pickup passes through an amplifier etc. not shown, and is subjected to removal of the DC component thereof in a DC-cut section configured by a capacitor 30. The detected voltage from which the DC component has been removed is subjected to binarization in a comparator 31. As described before, the data recorded on the optical disk is substantially free from the DC component due to a variety of modulations. Thus, if the slice level in the comparator is adequate, the output of integration of the results of comparison assumes zero. On the other hand, since the readout signal has a limited frequency band due to the frequency characteristic between the optical head and the medium, there arise a deviation in the duty if the slice level is deviated, whereby the output of integration of the results of comparison can be detected as a value that has a polarity depending on the deviation of the slice level and the polarity. Thus, a resistor 32 and a capacitor 33 are used for integration, and the slice level is fed back to the comparator 31 via a buffer 34, and controlled automatically to an adequate slice level.
A technique for correcting the deviation of DC level and suited to a digital configuration is described in Patent Publication JP-2007-059018A. This technique will be described with reference to FIG. 10. A readout signal is A/D-converted in an A/D converter 10 at the timing of a clock signal output from a PLL circuit 16, and delivered to a Viterbi detector 13 via an offset corrector 20 and an equalizer 12. An error-signal generator 115 generates an equalization error based on the detected data from the Viterbi detector 13 and the output from the equalizer 12. The error-signal generator 115 outputs to the offset corrector 20 an equalization error, which is in the vicinity of polarity inversion of the readout signal, out of the equalization errors generated therein. The offset corrector 20 integrates the equalization errors in the vicinity of the polarity inversion of the readout signal, to control the offset amount so that the integrated value assumes zero. Since the deviation of DC level is directly added to the equalization error, an offset correction can be achieved with a higher accuracy at a higher speed. This allows the Viterbi detector 13 to exercise the detection performance thereof at a maximum.
Since the readout signal in a detection system that requires a most-likelihood detector has a lower resolution or lower SNR, a large amount of noise is added thereto in the form of jitter. Thus, for detecting the slice level with a higher accuracy from the result of slicing in JP-1995-296386A, a long-time integration is needed. This incurs the problem that the accuracy of the slice level or the control band is impaired. In addition, upon configuration of a digital circuit, it is needed to increase the sampling rate of the A/D conversion for raising the time resolution, and to employ a larger number of quantization bits.
On the other hand, use of the technique described in JP-2007-059018A solves the problem of the tracking accuracy and tracking bandwidth of the offset. However, the readout signal from the optical disk is involved with a damage or dust on the medium surface and a minute defect etc. on the recording layer without an exception. In general, these defective areas appear together with a reduction in the amplitude or DC-level fluctuation in the readout signal. In the information detector described in JP-2007-059018A, a deviation of the sampling phase may occur upon passing through the defect. The most-likelihood detector, which performs detection operation on the premise that the sampling phase is correct, detects wrong detection data in the state of deviation of the sampling phase. If the thus detected data is used to generate the error information for the offset correction, the deviation of the DC level is accelerated. This deviation of DC level is fed back to the phase comparator in the PLL, to accelerate the deviation of the sampling phase. As a result, as exemplified in FIG. 11, the deviation of DC level occurs just after passing through the defect, to stabilize the phase deviation at π. Thus, the defect that can be originally corrected using the ECC without a problem eventually appears as a burst error that cannot be corrected.
JP-2007-059018A describes that, after performing judgment as to synchronization of the PLL, the offset correction operation is held or initialization is performed if out-of-synchronization is judged. However, if the SNR of readout signal is lower, it is difficult to judge the synchronous state in a short period of time from the state of phase comparison in the PLL. Thus, it is general to use the interval between the synchronizing flags, for example, in the detected data pattern as an index, whereby the judgment as to the synchronization consumes a significant time period. This time lag causes a large number of errors during the holding or initialization operation. In addition, even if the judgment is performed within the time limit, there is no guarantee that the sampling phase is correct after returning to the ordinary control, thereby causing a possibility of deviation of the DC level. In short, there is a problem in the stability against the deviation of PLL phase or DC level after passing through the defect.